The present invention relates channel distortion estimation, and more specifically to channel distortion estimation of a communications medium of communications system. Even more specifically, the present invention relates to amplitude and phase distortion estimation of a communications medium of a relatively time-invariant communications system.
In a communications system, signals comprising data are typically transmitted from a transmitter to a receiver via a communications medium or communications channel contained within a communications link. The transmitter modulates and transmits these signals at a specified modulation type (e.g. QPSK, 16-QAM, and 64-QAM) and at a specified data or signaling rate (e.g. 160 k bits per second) within the communications medium. Typically, the communications medium (also referred to simply as a medium) has a particular range of frequencies or bandwidth, such as from 5 MHZ to 42 MHZ, that the signals travel at over the communications link. Additionally, the medium also refers to the physical path which the signal travels over from a transmitter to a receiver.
As these data-bearing signals propagate over the medium of the communications link, the signals experience distortion such that the signals being received at a corresponding receiver are altered from their transmitted form depending on noise levels, non-linearities, time delays and reflections that are all frequency and medium dependent upon the signals within the medium, for example. Specifically, the amplitude and phase of the signals are distorted, which is referred to in the composite as medium dependent channel distortion (also referred to as xe2x80x9cchannel distortionxe2x80x9d. If the channel distortion of signaling over a particular medium is within an acceptable bit error rate, for example, the receiver demodulates the signal and extracts the data from the signal. Disadvantageously, if the channel distortion is too great or is above an acceptable bit error rate, the receiver will demodulate the signals and potentially misinterpret the information or data carried therein.
Knowledge of the individual components of the channel distortion, i.e. the amplitude distortion component and the phase distortion component, for a given medium provides the transfer function of the medium. The transfer function is commonly defined as the ratio of the output to the input of the system as a function of frequency, where the altered output as related to the input is commonly degraded due to medium dependent channel distortion.
In a relatively time-invariant communications system, i.e. the transmitter and the receiver are relatively fixed in location with respect to one another, it is advantageous to know the transfer function (i.e. the amplitude distortion and the phase distortion) of a particular medium of a communication link. With such knowledge, persons skilled in the art may make determinations as to the specific types of signaling that the particular medium may support. For example, it can be determined what SNR (Signal-to-Noise) capability or grades of service, i.e. what modulation levels and signaling rates, are supportable by the medium. Additionally, it can be determined whether a specific fixed rate transmitter and corresponding fixed rate receiver, each configured for a certain modulation level and signaling rate, will be supported by a particular medium. Furthermore, it may be desirable to determine if a medium that is currently used by a transmitter and receiver sending signals at a specified modulation level and signaling rate could support signals at a higher or lower modulation level and a higher or lower signaling rate. Thus, it is desirable to determine the transfer function of a given medium.
Currently, in order to determine whether a particular medium may support signals at a specified modulation level and signaling level, a transmitter and a receiver that send test signals at the specified modulation level and signaling rate are connected in the communications path and tested. Disadvantageously, this approach is not desired since the devices must be physically connected in the communications path. This may be time consuming and inefficient, for example, if the transmitter and receiver are located physically at a distance from each other, or if one of the transmitter or receiver is located within a subscriber""s residence. Furthermore, if the medium already is used to provide an existing service, for example, to a subscriber, the existing service would have to be interrupted for the duration of the testing of the medium. This may further serve to inconvenience or irritate the subscriber using the existing service.
Alternatively, an adaptive bandwidth and signaling rate scan receiver may be used in the communications path that can switch between higher and lower modulation levels and signaling rates. The HP89441 VSA (Vector Signal Analyzer), made by Hewlett Packard, is an example of such a device. Likewise, an adaptive test signal source that transmits the test signals is needed at the transmit side. Thus, fixed bandwidth and signaling rate transmitters and receivers must be replaced by expensive adaptive transmitters and receivers. Again, such dedicated adaptive equipment must be connected in the communications path and could inconvenience subscribers, as well as interrupt any existing services provided over the medium for the duration of the testing. Such adaptive equipment may already exist in the communications path; however, such equipment is very costly and inefficient.
Additionally, in order to test the medium, the test signaling transmitted over the medium by the dedicated equipment, whether adaptive or not, occupies the entire bandwidth of interest, i.e. occupies the entire medium having a given bandwidth. Disadvantageously, in a system including adaptive equipment, since the test signals occupy the entire medium under test, there may be a loss of throughput of an existing service being provided over the medium, even though additional equipment is not required because the existing equipment is adaptive.
Furthermore, with regard to estimating a transfer function of the medium, and since the test signals are transmitted over the entire spectrum of the medium being tested, the measured signal at the receiver provides a gross channel distortion estimate of the medium. This gross channel distortion estimate is inherently difficult, if not impossible, to separate into the individual components of the phase distortion and the amplitude distortion that are required for the transfer function. Thus, connecting dedicated equipment, whether adaptive or not, does not allow the estimation of the transfer function of the medium.
Thus, since the transfer function of the medium is not determined, the medium must be tested at each modulation level and signaling rate of interest to see what levels or grades of service the medium will support. In contrast, if the transfer function is known, the modulation levels and signaling rates supportable by a particular medium may be determined using known signal processing simulations, such as xe2x80x9cSPWxe2x80x9d designed by Cadence or xe2x80x9cSystem View by Elanixxe2x80x9d designed by Elanix, Inc.
Alternatively, other techniques may be used to determine the transfer function of a particular medium, for example, by using a network analyzer. A network analyzer is a two-port system that must be coupled to the transmit and receive end of the communications path. Once connected, the network analyzer sends test signals over small frequency portions of the medium to be tested. The receiver end measures and records amplitude and phase differences as viewed from the received sine wave in comparison to the transmitted sine wave and then analyzes these amplitude and phase relationships over the frequency range of the communications medium. Disadvantageously, again an existing service using the medium must be interrupted during the duration of the testing. Furthermore, since the network analyzer is a two-port system, both the transmit and receive end of the communications path must be coupled to the network analyzer. In communication s systems in which the transmit and receive ends are physically located at a distance from each, e.g. in a hybrid fiber/cable (HFC) system in which the communication medium may be on fiber and cable from the subscriber to a headend, it is impractical to string additional testing cables reaching many miles to attach both the transmit and receive end to the network analyzer.
Thus, a technique is needed to test a particular medium to estimate the phase and amplitude distortion of the particular medium, i.e. determine its transfer function, which can be used to interpolate what grades of service, i.e. modulation levels and signaling rates, are supportable by the particular medium without obtrusive test signaling that interrupts existing services or requiring dedicated equipment, such as adaptive transmitters and receivers, in the communications path.